Obsessive-compulsive disorder affects 2% of the population and produces substantial morbidity worldwide. It is often unresponsive even to optimal psychotherapy and pharmacotherapy. New insights into the neurobiology of the disorder, and the pharmacological strategies to which they may lead, are urgently needed. Convergent evidence suggest that dysregulation of the neurotransmitter glutamate may contribute to OCD; glutamate-modulating medications may therefore be of benefit in some of the 30% of cases refractory to standard treatment. Genetic associations and cerebrospinal fluid (CSF) findings supporting this hypothesis have been reported, by our group and others. Magnetic resonance spectroscopy (MRS) allows noninvasive measurement of glutamate in defined anatomical regions and thus offers the unique capacity to investigate dysregulation of the neurotransmitter in the specific circuit associated with OCD symptomatology. MRS studies have suggested glutamate abnormalities in the anterior cingulate cortex (ACC), the striatum, and the orbitofrontal cortex. However, these studies have been performed at low field strength and have been unable to dissociate glutamate from glutamine (reporting instead a compound measure, Glx, and have not generally replicated one another. The clarity possible through state-of-the-art glutamate measures at higher field strength is badly needed to refine our understanding of glutamate dysregulation in OCD. We here report a pilot study measuring glutamate and glutamine in the anterior cingulate cortex. Spectral fitting using a simulated basis set on data collected from a single midline voxel over the ACC in a 4T MRS scanner revealed reduced glutamate, but normal glutamine, only in euthymic OCD patients. Depressed OCD patients had normal glutamate levels. This unexpected effect suggests a pathophysiological interaction between these frequently comorbid disorders. It also may be a source of variability contributing to the failures of replication among published reports of Glx. We propose to replicate and expand these findings. First, we will replicate our pilot study in a larger cohort, tis time also including unmedicated depressed patients as a comparison group to better elucidate the interaction between diagnoses. Second, we will perform glutamate and glutamine measures in the striatum, another structure implicated in OCD in which Glx abnormalities (elevations, in this case) have been inconsistently reported. We will then investigate the ability of these measurements of brain neurochemistry to predict the response to pharmacotherapy; such an association would identify glutamate measures as a potential clinically useful biomarker to help guide therapeutic choices. This series of state-of-the-art spectroscopic investigations will elucidate the region-specific dysregulation of excitatory neurotransmission in OCD, importantly refining our understanding of the neurochemistry of the disorder while probing the potential utility of a novel and potentially clinically useful biomarker.